The present invention relates to a method and apparatus for monitoring a liquid for which the inclusion of impurities such as particles is disadvantageous, typified by ultrapure water that is, for example, used in semiconductor manufacturing, and for detecting the size of such particles.
In recent years, ultrapure water has become essential to wafer cleaning in semiconductor production processes. There are cases in which water includes impurities such as ions and colloidal particles, microbubbles and dissolved air, and particles (foreign particles) such as silica, cellulose acetate, and Teflon (registered trademark) polymer.
A measuring device that monitors the state of ultrapure water refined by a function membrane can be relatively easily realized by measuring conductivity with respect to ions. Also, in the case of air, since the active agent oxygen is the cause of defects, the measuring device can be realized by established technology for eliminating the actual influence of the air by removing all of the dissolved air with a degassing membrane, and thereafter reducing the partial pressure of oxygen by dissolving carbon dioxide that has been refined for static protection into the ultrapure water.
However, the technology that is farthest behind and has not caught up with the speed of progress in function membranes is an apparatus (particle sensor) for monitoring “foreign particles” in ultrapure water. In particular, it is no exaggeration to say that there is no product whatsoever that is suitable for inline constant monitoring for processes, and there is demand for a particle sensor that is easy-to-handle and inexpensive.
Various methods related to the detection of microparticles in water are known.
Specifically, there is a method of detecting the characteristics of microparticles included in a liquid by shining light into a specimen liquid and measuring the attenuation of the transmitted light or the amount of scattered light that has leaked laterally.
FIG. 26 is a diagram showing the configuration of a conventional scattered light-type detection apparatus 80.
As shown in FIG. 26, the scattered light-type detection apparatus 80 is composed of a laser light source 81, a lens 82 that converts light into parallel light, a transparent flow cell 83 that has rectangular cross section for allowing the flow of a reagent, and a light receiving element 84 that receives scattered light.
Light that has been emitted from the laser light source 81 is converted into parallel light by the lens 82, and incidences on the flow cell 83 while maintaining a uniform light intensity. If the light strikes a microparticle included in the reagent, scattered light is produced in an amount corresponding to the particle size of the microparticle, with a vector characterized by the particle size. The light receiving element 84 measures the intensity of the exiting scattered light and the angle relative to the flow cell 83, and the source microparticle size is determined from the obtained data.
With the scattered light-type method, due to the principle thereof, the vector of the exiting scattered light is a very important element. For this reason, if the flow cell 83 is given a round-tube shape, it will have the characteristics of a lens, and therefore the scattered light will be bent, and accurate measurement will not be possible. It is therefore necessary to use the rectangular cross section flow cell 83, which is very difficult to work, and this is a cause for a rise in cost and difficulty in maintenance.
Patent Document 1 discloses a detection method that does not have such shortcomings of the scattered light-type method. Specifically, Patent Document 1 discloses a method of focusing a light beam from a light source in a flow channel for a detection-target liquid, thus producing a radiant light beam, and then detecting, with use of a light detecting device, diffraction fringes that appear when particles pass by in the vicinity of the focal point. The time required for the particle to traverse the light beam is measured, and the dimensions of particles in the liquid are discriminated based on the relationship that the time from the appearance to the disappearance of diffraction fringes appearing due to particles passing by in the vicinity of the focal point is short, and the time from the appearance to the disappearance of diffraction fringes appearing due to particles passing by at positions away from the focal point is long, and also based on the relationship that the detectable passing position is limited according to the particle size of the particles. Accordingly, measuring the intensity and vector of light is not necessary as in conventional technology.
Patent Document 2 also discloses a method of measuring the particle size of microparticles. According to this method, a first measurement value that is correlated with the particle size of a microparticle included in a specimen fluid is detected from the output of at least one photoelectric conversion element, and a second measurement value that is correlated with a microparticle passing position is detected using a time difference between detections made by a pair of photoelectric conversion elements. Then, the first measurement value is corrected using the second measurement value, and the particle size of the microparticle is extracted.
However, in the case of using the method disclosed in Patent Document 1, there is a large constraint condition in actual use due to reasons particular to the principle of the method. When a reagent outside the constrain condition is measured, there is a problem with the accuracy of the display.
Specifically, the above method is effective when there is a sufficiently large number of particles included in the detection-target liquid, that is to say, in cases in which the detection target is tap water, lake water, or the like, but sufficiently precise detection is impossible for liquids in which the amount of particles contained is extremely small, such as ultrapure water. It should be noted that although tap water contains roughly several tens of thousands of microparticles per mL, ultrapure water used in wafer cleaning is said to contain microparticles on the order of 1/mL or less.
Since statistical processing is used in the method disclosed in Patent Document 1, it is impossible to ensure that the number of signals that are the basis of calculation is stochastically sufficient, and obtaining a stochastically significant number of signals requires too long of a measurement time and is not suitable for practical use. Also, since the calculation based on detected signals progresses in order from large particles to intermediate-size particles to small particles, a corresponding error accumulates.
Also, given that the method disclosed in Patent Document 2 corrects detected signal values that are correlated with particle sizes with use of detected passing position information, the characteristics of the detected signal values that in principle are correlated with particle sizes appear intensely, and therefore it is impossible to perform sufficient correction, and this method is not suited for the accurate detection of particle sizes. Moreover, Patent Document 2 does not disclose a correction method for accurately detecting particle sizes, and in view of this point as well, the accurate detection of particle sizes is impossible.
Patent Document 1: Japanese Patent No. 3745947
Patent Document 2: Japanese Patent No. 3301658